Organic esters are important intermediates in chemical and pharmaceutical industries, and they are mostly produced by acid-catalyzed esterification reactions. The esters of bio-based organic acids fall into the category of benign or green solvents and are promising replacements for halogenated petroleum-based solvents in a wide variety of applications. Citric acid can be esterified with alcohols like ethanol and n-butanol through a series of reactions to yield tri-ethyl citrate (TEC) and tri-n-butyl citrate (TBC). TEC and TBC are used as nontoxic plasticizers in toys, medical products (e.g., as enteric coatings for controlled release drug delivery systems), printing ink coatings, cosmetics, and other applications. These plasticizers are also suitable as food additives such as whipping agents for dried egg whites, food flavorings, or food packaging materials. The global plasticizers market has been estimated at around 11 billion pounds per year; according to 2003 statistical data, the U.S. share of this market is 2.4 billion pounds.
TBC is widely used as a solvent and plasticizer in the field of food additives, food contact materials, medical products and cosmetics due to its high boiling point and lower molecular weight. At high temperatures TBC is a stable compound and act as biodegradable polymer. Since, TBC has been derived from the substances which are easily available from renewable resources by fermentation process; therefore TBC is an environmentally friendly plasticizer.
Industrially, TBC have been produced by the esterification reaction of citric acid with butanol with catalytic process. Traditionally, conc. sulphuric acid was used as catalyst. But catalytic esterification process becomes more complex due to increase of number of processes such as deacidification, neutralization, washing, drying, bleaching, and filtration process. Other than complexity, serious equipment corrosion, long reaction period large investment in equipment, poor reaction selectivity, low product quality, high-cost, colour depth, by-products makes more complex post-processing operations, wastewater capacity and serious environmental pollution. The cost of conc. sulphuric acid is low and having high catalytic activity as compared to the other catalyst used. Esterification of carboxylic acids with alcohols in presence of acid catalysts has been extensively investigated. Typical homogenous catalysts like H2SO4 and p-TSA are used but due to their miscibility with the reaction medium, however separation becomes a serious problem. In conventional manufacturing of tributyl citrate, various catalysts are used for esterification process. These include both homogeneous and heterogeneous catalysts. Major catalysts used are as are H2SO4, PTSA (para toluene sulphonic Acid), cation exchange resins, titanate, solid acid catalysts, ionic liquid catalysts etc.
Chinese patent application no. 104478716 discloses a process for the synthesis tributyl citrate plasticizer. The synthesis process comprises steps as follows: (1) an esterification reaction; (2) neutralization and washing; (3) distillation; (4) decolorization.
Chinese patent application no. 104892418 discloses a synthesis method for the preparation of acid tributyl citrate. The synthesis method comprises the following steps of stirring and reacting at the room temperature to obtain a product citric acid tributyl citrate by utilizing a condensing agent o-benzotriazole-N,N,N′,N′-tetramethyluronium tetrafluoroborate (abbreviated as TATU) by means of esterification reaction of citric acid and n-butyl alcohol.
US patent application no. 20060252956 discloses a process for producing organic acid di- or tri-esters, particularly citric acid tri-esters, with the available acid groups esterified using countercurrent reactive distillation using acid catalysts in a structured packing. In the reactive distillation an organic acid di- or tri-ester is formed by chemical reaction and purified to its final state within the single column. Organic acid di- or tri-esters are produced at relatively low cost, with less waste production in by-products of the reaction, and in a less complicated manner than prior processes. Organic acid di- and tri-esters have uses as solvents, as plasticizers and in conversion products.
Article titled “Reaction kinetics of the catalytic esterification of citric acid with ethanol” by AK Kolah et al. published in Ind. Eng. Chem. Res., 2007, 46 (10), pp 3180-3187 reports reaction kinetics for the reversible esterification reaction of citric acid with ethanol to form tri-ethyl citrate via mono-ethyl and di-ethyl citrates. The reaction was studied in batch isothermal experiments, self-catalyzed homogeneously by citric acid and the formed mono- and di-ethyl citrates, and heterogeneously catalyzed by macroporous Amberlyst-15 ion-exchange resin catalyst.
Article titled “Continuous process for esterification of citric acid: Formation of triethyl citrate” by Asthana et al. published in ACS National Meeting Book of Abstracts, 2007 reports a continuous process for the formation of triethyl citrate using cationic exchange resins as catalysts in a reactive distillation column and batch reactor. Vapor-liquid equilibrium studies for important binary mixtures have also been conducted. In a standard batch reaction, equilibrium was achieved after 16 hours; 99% of the citric acid was converted to a mixture of monoethyl, diethyl, and triethyl citrate, with a batch triethyl citrate selectivity of 64%. Kinetic parameters that include both ion-exchanged catalyzed reactions and self-catalyzed reactions were generated by non-linear regression of batch experimental data in MATLAB.
U.S. Pat. No. 2,523,792 discloses citric acid ester compositions, and more particularly to compositions including esters of citric acid. The monoisopropyl citrate is prepared by using Equal parts of U. S. P. citric acid and commercial 99% isopropyl alcohol are heated together under reflux, without catalyst for 118 hours at 92° C.
PCT application no. 2003008369 discloses a method for the production of citric acid esters of citric acid and monovalent straight or branched chain alcohols having a chain length of 4 to 10 carbon atoms. The process for the catalytic esterification of citric acid with straight or branched chain 4-10 carbon atoms with alcohols comprises three stages: a) in the first stage, the reaction mixture is heated to temperatures ranging from 100 to 130° C. and esterification takes place by means of auto-catalysis up to a conversion rate of 80 to 90%; b) in the second stage the catalyst is added, the reaction temperature is adjusted between 100 and 300° C. depending on the stability of the respective ester, and esterification continues up to a conversion rate of 90 to 95° C.; and c) in the third stage the reaction temperature is maintained at the highest value reached in the second stage, anhydrous alcohol is optionally added depending on the respective alcohol components, the supply of inert gas is multiplied by three or six relative to the first two sections.
Article titled “Triethyl citrate synthesis by reactive distillation” by AK Kolah et al. published Ind. Eng. Chem. Res., 2008, 47 (4), pp 1017-1025 reports a continuous reactive distillation process is proposed for the synthesis of triethyl citrate from citric acid and ethanol in the presence of macroporous Amberlyst 15 ion-exchange resin catalyst.
Homogeneous catalysts offer a number of important advantages over their heterogeneous counterparts. For example, all catalytic sites are accessible because the catalyst is usually a dissolved metal complex. Furthermore, it is often possible to tune the chemo selectivity, region selectivity, and/or enantio selectivity of the catalyst. Despite these advantages, many homogeneous catalytic systems have not been commercialized because of one major disadvantage compared with heterogeneous catalysts the difficulty encountered when trying to separate the reaction product from the catalyst and reaction solvent. This problem arises because the most commonly used separation method, distillation, requires elevated temperatures unless the product is very volatile. Most homogeneous catalysts are thermally sensitive, usually decomposing below 150° C. Other conventional processes such as chromatography or extraction also lead to catalyst loss. However, catalyst separation is tedious process involving distillation & separation process. These processes might not give 100% recovery of the catalyst which increases cost of the process. Many times catalyst is highly soluble in water which causes water pollution.
The catalytic processes that are known in the prior art have following limitations i) separation of catalyst (H2SO4 and P-TSA) from reaction mixture and its reuse is difficult ii) many impurities was formed in the presence of catalyst (H2SO4 and p-TSA) iii) catalytic process gave yellowish and brownish colored TBC product.
Therefore, there is need to develop a cost effective, eco-friendly process for the synthesis of TBC with high purity and yield. Accordingly, the present inventors provide an eco-friendly, cost effective, autocatalytic process for the synthesis of tributyl citrate (TBC) with high yields and purity.